The latest additions to the US build-up are thought to be mainly army and air force units.

The forces, which are not expected to leave immediately, will ultimately give US President George W Bush a wide set of military options.

Our correspondent says these latest deployment orders are confirmation that the build-up has moved into a significant new phase.

Having spent weeks and months painstakingly stocking up equipment and supplies in the region, the Pentagon is now beginning to move large numbers of combat troops and air power - the key elements of any attack force.

It is believed that the Pentagon is ultimately aiming for a force of a 250,000.

Ark Royal leaves

The 35,000 reinforcements ordered on Friday include marines, who would spearhead any possible invasion force.

The troops are being sent to the Gulf in three amphibious warfare ships.

The American deployment comes as the British aircraft carrier Ark Royal set sail for the Gulf in the biggest British naval deployment since the Falklands War.

The UK Ministry of Defence insists that the role of what will be 8,000 troops in the Gulf has not been decided, but it is clear that it could assist the US in some form of amphibious assault on Iraq.